These studies continue to assess the distribution of [125I]RTI-55-labeled dopamine and serotonin uptake sites in the rat brain. Cocaine exerts its addictive properties through the blockade of DA uptake sites. Studies have been done to characterize the specific effects of cocaine on the brain, using ligands which bind at more than the DA uptake sites. Effort is being made to develop ligand that are more specific to DA uptake sites. One of these ligands is [125I]RTI-55. We have determined the specific distribution of these sites in the rat central nervous system in a number of studies. In addition, we evaluated the effects of 6-OHDA lesions in the nigrostriatal DA pathway. Unilateral lesions of the nigrostriatal dopaminergic pathway were performed by the stereotaxic application of 6-OHDA in the caudate-putamen. 6-OHDA caused marked decreases in total [125I]RTI binding sites in the NAc and the CPu. Further analyses indicated that these decreases corresponded to changes in both 5-HT and DA uptake sites. These studies also helped to demonstrate that [125I]RTI-55 bind to 5-HT sites in the caudate-putamen and in the frontal cortex of rats. These experiments stress the important of both biochemical and lesion studies in order to characterize the anatomical distribution of monoaminergic uptake sites in the rat brain.